Sizing a UPS correctly - VA, watts and runtime explained
A UPS is limited by two ratings at once: apparent power in VA and real power in watts. This guide shows how to calculate the right size from your load, the power factor and the required runtime, and how to plan sensible headroom.
View UPS systemsWhy are UPS units rated in both VA and watts?
Every UPS carries two ratings: apparent power in volt-amperes (VA) and real power in watts (W). VA limits the current the electronics and inverter can deliver, while watts limit the power actually turned into heat and work. Your load must exceed neither of the two values.
The relationship is: watts = VA × power factor (cos φ). Older units often used a factor of 0.6, while modern line-interactive and online UPS reach 0.8 to 1.0. A 1500 VA UPS with a factor of 0.9 therefore delivers around 1350 W.
How do you calculate the required UPS capacity?
First add up the real power of all connected devices in watts. Use the rating plate or power supply figures, not the maximum fuse rating, which is usually much higher. From the total watts and the power factor of your equipment you derive the VA requirement.
- Sum the real power: add every load in watts.
- Derive the VA need: required VA = watts / the load cos φ.
- Add headroom: 20 to 25 % for inrush peaks and later expansion.
- Pick a target load: run the UPS at 70 to 80 % of its rating.
- Check both limits: the chosen UPS must cover watts and VA at once.
Devices with switch-mode supplies, motors or laser printers draw brief inrush peaks that can be several times the continuous power. Laser printers therefore do not belong on a tightly sized UPS. Plan extra headroom for such loads.
How do you plan runtime and reserves?
Runtime depends on battery capacity and the actual load, not on the nameplate rating. The higher the load, the shorter the runtime - and the relationship is not linear, because lead-acid batteries deliver less usable capacity at high current (the Peukert effect).
For a safe shutdown, 5 to 10 minutes are often enough. To ride through longer outages completely, you need external battery modules or a larger UPS class. Always calculate runtime for the real load, not the nameplate, and factor in the capacity loss of ageing batteries.
Matching replacement batteries and external modules for longer runtime.
Read the guideFrequently asked questions
What do VA and watts mean on a UPS?
VA is apparent power and limits the current available, watts is real power. The relation is watts = VA × power factor. Your load must exceed neither value.
How much headroom should you plan?
A common figure is 20 to 25 % headroom over the calculated load. This leaves room for inrush peaks and later expansion, and keeps the UPS at a comfortable 70 to 80 % load.
Why does runtime drop faster than the load rises?
Lead-acid batteries deliver less usable capacity at high discharge current (the Peukert effect). Doubling the load does more than halve the runtime, it often falls disproportionately.
Can I connect a laser printer to the UPS?
Better not to a tightly sized UPS. Laser printers draw high inrush peaks that are several times the continuous power and can overload the UPS.
Looking for the right UPS size?
We help you size by VA, watts and required runtime - including battery reserve and expansion headroom.
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Sized against VA and watts at once.
Headroom included
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